Extend Spotify plugin to obtain (popularity and audio features) track attributes

[arsaboo]

This PR extends the spotify plugin to add another command beet spotifysync to obtain all the audio features available for a track. This includes the popularity information and audio_features. The following audio_features are included: acousticness, danceability, energy, instrumentalness, key, liveness, loudness, mode, speechiness, tempo, time_signature, valence. See details here.

This command only processes tracks that already have the spotify_track_id (which was introduced in #4348). So, the workflow will be to run beet import to obtain the spotify_track_id. This information can then be used to run beet spotifysync.

Fixes #4347 #3578 #4094

optional config:

spotify:
    client_id: SPOTIFY_CLIENT_ID
    client_secret: SPOTIFY_CLIENT_SECRET

To Do

• Documentation. (If you've add a new command-line flag, for example, find the appropriate page under docs/ to describe it.)
• Changelog. (Add an entry to docs/changelog.rst near the top of the document.)
• Tests. (Encouraged but not strictly required.)

[arsaboo]

@sampsyo I was thinking that instead of force, we should have separate overwrite flags. audio_features are static and may not have to be updated, whereas popularity is dynamic and users may want to update them. We don't have to tie them together. In addition, we may want to add other attributes later. Not everyone may want all the attributes, so it may be better to decouple them. Here's the config that I have in mind (with defaults indicated):

spotify:
    client_id: SPOTIFY_CLIENT_ID
    client_secret: SPOTIFY_CLIENT_SECRET
    popularity: yes
    popularity_overwrite: no
    audio_features: no
    audio_features_overwrite: no

[sampsyo]

Awesome; looking good so far! Here are a few suggestions/questions.

Do you also think you might be able to add to the docs for the plugin to describe the new command? A changelog entry would also be super helpful.

[arsaboo]

Thanks for the review @sampsyo. Addressed all the comments and updated the documents/changelog.

[JOJ0]

Just here to say woohoo! Very nice feature! Thanks to the submitter 💣💥

[arsaboo]

Thanks @JOJ0

I have a few additional features in mind once this PR gets merged.

[wisp3rwind]

Looks good to me! @sampsyo all your previous comments have been addressed, any other concerns? Or can we go ahead and merge this?

[sampsyo]

Nice!! This looks really great. Thank you for seeing this through!!